Researchers who used PET with F-18-fluorofuranylnorprogesterone reported in the Journal of Nuclear Medicine that T47D human breast cancer cells and mouse models with T47D tumor xenografts that were treated with estrogen had elevated tracer uptake after 48 hours and after 48 hours and 72 hours, respectively, which was associated with higher expression and spread of the progesterone receptor protein. The study also found no significant preferential tracer binding between PR-A and PR-B isoforms in either tumor xenografts or PR isoform-specific cell lines, and researchers said the approach may provide earlier measurement of hormone therapy efficacy in breast cancer.